Question

A program at host A sends a UDP datagram carrying 4500 octets1 of data (or payload) to its counterpart at host C. In between A and C is router B. The MTU between A and B is 1500 octets while between that B and C 1320 octets. Show IP packets and fragments along the path. For each IP packet and fragment, show key header fields like Total Length, Identification, More Flag, and Fragmented Offset

Answer 1

Solution:

Given,

=>Total data or payload size = 4500 octets

=>A.....B......C

=>MTU between A and B = 1500 octets

=>MTU between B and C = 1320 octets

Explanation:

=>Lets consider minimum header size of IPV4 header = 20 bytes

=>1 octet = 1 byte

Datagram:

20 bytes

4500 bytes

Header              Data

MTU in between A and B:

20 bytes

1480 bytes

=>As MTU in between A and B is less than the datagram size so fragmentation will be there.

Fragmentation between A and B:

Fragment 1:

=>Total length = 1500 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 1 because fragment 2, 3 and 4 will be following.

=>Fragment offset = 0

20 bytes

1480 bytes

   Header            Data

Fragment 2:

=>Total length = 1500 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 1 because fragment 3 and 4 will be following.

Calculating fragment offset:

=>1480/8 = 185

=>Fragment offset = 185

20 bytes

1480 bytes

   Header            Data

Fragment 3:

=>Total length = 1500 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 1 because fragment 4 will be following.

Calculating fragment offset:

=>1480/8 = 185

=>Fragment offset = 370

20 bytes

1480 bytes

    Header            Data

Fragment 4:

=>Total length = 1500 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 0 because no fragment will be following.

Calculating fragment offset:

=>1480/8 = 185

=>Fragment offset = 555

20 bytes

20 bytes

   Header            Data

Fragmentation in between B and C:

MTU in between B and C:

20 bytes

1300 bytes

    Header            Data

Fragment 1 of fragment 1:

=>Total length = 1316 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 1 because other fragments are following

Calculating fragment offset:

=>Fragment offset = 0

20 bytes

1296 bytes

   Header            Data

Fragment 2 of fragment 1:

=>Total length = 204 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 1 because other fragments are following

Calculating fragment offset:

=>1296/8 = 162

=>Fragment offset = 162

20 bytes

184 bytes

   Header            Data

Fragment 1 of fragment 2:

=>Total length = 1316 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 1 because other fragments are following

Calculating fragment offset:

=>Fragment offset = 185

20 bytes

1296 bytes

   Header            Data

Fragment 2 of fragment 2:

=>Total length = 204 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 1 because other fragments are following

Calculating fragment offset:

=>1296/8 = 162

=>Fragment offset = 347

20 bytes

184 bytes

   Header            Data

Fragment 1 of fragment 3:

=>Total length = 204 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 1 because other fragments are following

Calculating fragment offset:

=>Fragment offset = 370

20 bytes

1296 bytes

   Header            Data

Fragment 2 of fragment 3:

=>Total length = 204 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 1 because other fragments are following

Calculating fragment offset:

=>1296/8 = 162

=>Fragment offset = 532

20 bytes

184 bytes

   Header            Data

Fragment 4:

=>Total length = 40 bytes

=>Let say identification = ID because it remains same for a datagram throughout.

=>More flag = 0 because no other fragments are following

Calculating fragment offset:

=>Fragment offset = 555

20 bytes

20 bytes

   Header            Data

I have explained each and every part with the help of statements attached to it.